What are the differences (if any) between ES6 arrow functions and functions bound with Function.prototype.bind?

Question

It seems to me that, in ES6, the following two functions are very nearly identical:

function () {   return this; }.bind(this);  () => {   return this; }; 

The end result seems the same: arrow functions produce a JavaScript function object with their this context bound to the same value as the this where they are created.

Obviously, in the general sense, Function.prototype.bind is more flexible than arrow functions: it can bind to values other than the local this, and it can bind any function's this at any point in time, potentially long after it is initially created. However, I'm not asking how bind itself is different from arrow functions, I'm asking how arrow functions differ from immediately calling bind with this.

Are there any differences between the two constructs in ES6?

Answer 1

There are no (significant) differences.

Well, okay, that's a little premature. There are three tiny differences unique to arrow functions.

1. Arrow functions cannot be used with new.

   This means, of course, that they do not have a prototype property and cannot be used to create an object with the classically-inspired syntax.

   new (() => {}) // TypeError: () => {} is not a constructor 

   This is probably for the best, though—the way new works would not make much sense with bound functions.

2. Arrow functions do not have access to the special arguments object that ordinary JavaScript functions have access to.

   (() => arguments)(1, 2, 3) // ReferenceError: arguments is not defined 

   This one is probably a little bit more of a gotcha. Presumably this is to remove one of JavaScript's other oddities. The arguments object is its own special beast, and it has strange behavior, so it's not surprising that it was tossed.

   Instead, ES6 has splats that can accomplish the same thing without any magic hidden variables:

   ((...args) => args)(1, 2, 3) // [1, 2, 3] 

3. Arrow functions do not have their own new.target property, they use the new.target of their enclosing function, if it exists.

   This is consistent with the other changes to remove "magically" introduced values for arrow functions. This particular change is especially obvious, considering arrow functions can't be used with new anyway, as mentioned above.

Otherwise, arrows are just like bound functions, semantically. It's possible for arrows to be more performant, since they don't have to carry around the extra baggage and since they don't need to be converted from ordinary functions first, but they're behaviorally exactly the same.

Answer 2

There are a few differences:

• Arrow functions cannot be constructed. While both arrow functions and bound functions both don't have a .prototype property, the former do throw an exception when called with new while the latter just ignore the bound value and call their target function as a constructor (with the partially applied bound arguments, though) on the new instance.

  function F() {} var f = () => {},     boundF = F.bind({}); console.log(new boundF(), new boundF instanceof F) // {}, true console.log(new f) // TypeError 

• Arrow functions do have lexical arguments, new.target and super as well (not only lexical this). A call to an arrow function does not initialise any of those, they are just inherited from the function the arrow function was defined in. In a bound function, they just refer to the respective values of the target function.

• Arrow functions don't actually bind a this value. Rather, they don't have one, and when you use this it is looked up like a variable name in the lexical scope. This does allow you to lazily define an arrow function while this is not yet available:

  class X extends Object {     constructor() {          var f = () => this, // works              boundF = function(){ return this; }.bind(this); //                                                    ^^^^ ReferenceError          super(); // initialises `this`          console.log(f(), f() == this); // {}, true     } } new X; 

• Arrow functions cannot be generator functions (though they can return generators). You can use .bind() on a generator function, yet there is no way to express this using an arrow function.